The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portions that are exposed become soluble so that they are removed in the developing process. Such a plate is referred to in the art as a positive-working printing plate. Conversely, when the portions of the coating that are exposed become hardened, the plate is referred to as a negative-working plate. In both instances, the image areas remaining are ink-receptive or oleophilic and the non-image areas or background are water-receptive or hydrophilic. The differentiation between image and non-image areas is made in the exposure process where a film is applied to the plate under vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation.
Various useful printing plates that can be either negative-working or positive-working are described, for example, in GB 2,082,339 (Horsell Graphic Industries) and U.S. Pat. No. 4,927,741 (Garth et al), both of which describe imaging layers containing an o-diazonaphthoquinone and a resole resin, and optionally a novolac resin. Another plate is described in U.S. Pat. No. 4,708,925 (Newman) wherein the imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. This imaging composition can also be used for the preparation of a direct laser addressable printing plate, that is imaging without the use of a photographic film.
Direct digital imaging of offset printing plates is a technology that has assumed considerable importance to the printing industry. The first commercially successful workings of such technology made use of visible light-emitting lasers, specifically argon-ion and frequency doubled Nd:YAG lasers. Printing plates with high photosensitivities are required to achieve acceptable through-put levels using plate-setters equipped with practical visible-light laser sources. Inferior shelf-life, loss in resolution and the inconvenience of handling materials under dim lighting are trade-offs that generally accompany imaging systems exhibiting sufficiently high photosensitivities.
Advances in solid-state laser technology have made high-powered diode lasers attractive light sources for plate-setters. Currently, at least two printing plate technologies have been introduced that can be imaged with laser diodes emitting in the infrared regions, specifically at about 830 nm. One of these is described in EP 573,091 (Agfa) and in U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. 5,351,617 (Williams et al), U.S. Pat. No. 5,379,698 (Nowak et al), U.S. Pat. No. 5,385,092 (Lewis et al) and U.S. Pat. No. 5,339,737 (Lewis et al). This technology relies upon ablation to physically remove one or more layers from the printing plate. Ablation requires high laser fluences, resulting in slower imaging and problems with debris after imaging.
A higher speed and cleaner technology is described in U.S. Pat. No. 5,340,699 (Haley et al), U.S. Pat. No. 5,372,907 (Haley et al), U.S. Pat. No. 5,466,557 (Haley et al) and EP-A-0 672 954 (Eastman Kodak) which uses near-infrared energy to produce acids in an imagewise fashion. These acids catalyze crosslinking of the coating in a post-exposure heating step. Precise temperature control is required in the heating step. The imaging layers in the plates typically comprise a resole resin, a novolac resin, a latent Bronsted acid and an infrared absorbing compound. Other additives, such as various photosensitizers, may also be included.
The use of dissolution inhibitor compounds having acid-cleavable C--O--C groups in positive-working printing plates is also known. Representative of such compounds are the ortho carboxylic acid esters described in U.S. Pat. No. 4,101,323 (Buhr et al) and the polyacetals described in U.S. Pat. No. 4,247,611 (Sander et al). These compounds prevent dissolution of normally alkali-soluble phenolic resins in alkaline developer solutions.
Moreover, the dissolution inhibitors are generally mixed with photolytic acid-generating compounds in the photosensitive layers of the printing plates. Upon imagewise exposure of the layers to visible or ultraviolet light, an acid is released from the photolytic acid-generating compound which then catalyzes the decomposition of the dissolution inhibitors in the exposed regions. When this occurs, the phenolic resins can then be dissolved in alkaline developer compositions to provide a positive image in the exposed regions.
A number of such imaging systems are known, as described, for example, in U.S. Pat. No. 4,101,323 (noted above), U.S. Pat. No. 4,247,611 (noted above), U.S. Pat. No. 4,421,844 (Buhr et al), U.S. Pat. No. 4,506,006 (Ruckert), U.S. Pat. No. 4,678,737 (Schneller et al), U.S. Pat. No. 4,840,867 (Elsaesser et al), U.S. Pat. No. 5,149,613 (Stahlhofen et al), U.S. Pat. No. 5,286,602 (Pawlowski et al), U.S. Pat. No. 5,314,786 (Roeschert et al) and U.S. Pat. No. 5,346,806 (Pawlowski et al). However, all of these systems require irradiation and acid generation from UV or visible light irradiation.
Laser imaging of layers containing acid-cleavable groups has been disclosed in U.S. Pat. No. 5,314,786 (noted above) in which krypton-fluoride lasers emitting in the deep UV are used to provide positive images. The imaging layers described in this patent contain O,N-polyacetals, polyhydroxystyrene binder resins, polyacetal dissolution inhibitors and hydroxyethoxytriazine acid generating compounds. An argon ion laser is described in U.S. Pat. No. 4,506,006 (noted above) for use with similar photosensitive printing plates. Such a laser would have an emission wavelength at either 351 or 488 nm. A similar composition is described in U.S. Pat. No. 5,149,613 (noted above) to provide negative images by floodwise exposure followed by imaging with a krypton laser emitting at 647 or 676 nm.
There remains a need for compositions containing phenolic resin dissolution inhibitors and thermochemical acid-generating compounds that can be easily laser imaged in the infrared or near infrared regions of the spectrum at moderate power levels to provide positive images.